There is a need for a process for reliably producing high quality, gravity cast, thin wall metal castings. This need is especially acute in the automotive industry where efforts to increase fuel economy require that attempts be made to reduce the mass of the automobile. Significant reductions in the mass of cast metal components such as automotive engine blocks, exhaust manifolds and the like could be obtained if dimensionally accurate, defect-free castings could be reliably and efficiently produced in high volume.
When we refer to thin wall castings, we mean castings having substantial wall surfaces as small as one to three millimeters in thickness. Frequently, the thin wall portions of such castings have a rounded cross-section (i.e., circular, elliptical, octagonal, etc.) and are no more than about 160 mm in diameter. Examples of such castings are tubes, engine exhaust manifolds, cylinder heads, engine blocks, pistons and the like.
The difficulty with producing thin wall castings arises from the need for cast hot molten metal to flow through extensive, relatively small cavity passages in an unheated mold. Any freezing of the metal before the cavity is completely filled will yield castings with nonuniform walls or castings with holes or other defects. There are existing commercial processes for the casting of thin wall iron and aluminum castings that provide some inducement to the flow of the cast metal to promote complete mold fill before solidification. In these practices, a suitably designed resin-bonded sand mold is prepared that suitably defines the thin wall portions of the casting. The mold is filled from the bottom utilizing a pump or a pressure differential to cause the molten metal to flow rapidly into the mold cavity to fill it before solidification occurs. In these practices, a continuous metal flow link must be made between a reservoir of molten metal and the mold. In one such practice, the reservoir is pressurized to cause the flow of metal toward the mold. In another practice, the mold is subjected to a vacuum to assist the flow of cast metal into the mold cavity. In other practices, both a vacuum in the mold and pressure on the reservoir are employed.
In each of these cases, it is necessary for the metal at the ingates to the casting cavity in the mold to freeze off before the mold can be removed from the reservoir from which the metal is contained. This means that appreciable solidification must occur before the next mold can be filled from the molten metal reservoir. This slows a casting line, decreasing production rates.
Such prior art practices have an additional disadvantage. They require special equipment to provide for pressurization of the molten metal reservoir or for containment of the mold in a vacuum chamber or both. In some practices, an electromagnetic pump is employed. Both of these metal flow inducing mechanisms represent a substantial capital investment as well as process complexity which add to the cost of castings produced.
It is an object of the present invention to provide a resin-bonded sand mold design that will accommodate the gravity pouring of molten metal so that thin wall, defect-free castings can be reliably, accurately and efficiently produced.
It is a further object of the present invention to provide a gravity-castable mold design that permits all portions of the thin wall mold cavity to be filled with hot metal at substantially the same temperature and the same time, thereby rapidly filling the whole cavity and minimizing the chances for premature metal freezing and faulty castings.
It is a still further object of the present invention to provide a resin-bonded sand mold that is specially adapted for the casting of thin walled ducts for fluid flow such as, for example, tubes, engine exhaust manifolds, cylinder heads and engine cylinder blocks.